Along with the spread of mobile instruments in recent years, sealed batteries of small size, light weight, and high energy density have been desired as a power supply for these mobile instruments. Of the sealed batteries, secondary batteries capable of charging and discharging, such as nickel-hydrogen storage batteries and lithium-ion secondary batteries, have been increasingly used in terms of economy. In particular, nonaqueous electrolyte secondary batteries represented by a lithium-ion secondary battery have been increasingly used due to their lighter weight and higher energy density than other secondary batteries.
However, gas is generated in the secondary battery from decomposition of the electrolyte in an overcharge state where current is supplied longer than normal at the time of charging, when exposed to high temperature, or in a short-circuit state where a large current flows due to misuse and failure of an instrument to be used, for example. The generation of gas causes an increase in battery internal pressure. When such overcharge, exposure to high temperature, or a short-circuit state continues, the battery internal pressure can further increase to reach a dangerous state. Therefore, particularly in the case of a nonaqueous electrolyte secondary battery, those provided with a safety valve for explosion protection have been frequently used.
The safety valve needs to reliably activate in order to prevent damage of an instrument and prevent fire accidents, for example. Therefore, as shown in Patent Document 1 below, for example, a cylindrical nonaqueous electrolyte secondary battery has the following configuration: a wound electrode assembly is formed by spirally winding a positive electrode plate and a negative electrode plate that are arranged to face each other with a separator therebetween into a shape having a hollow portion at the center, and is arranged in a battery outer can; and a cylindrical center pin is arranged in the hollow portion of the wound electrode assembly, so that gas generated by an abnormal state such as overcharge is guided to the safety valve via the center pin arranged in the hollow portion of the wound electrode assembly. The center pin is provided to prevent the hollow portion from being crushed and blocking a gas passage when the pressure caused by the gas generated inside the cylindrical nonaqueous electrolyte secondary battery is applied in a stacking direction of the positive electrode plate, the negative electrode plate, and the separator.
In this manner, in a cylindrical nonaqueous electrolyte secondary battery, safety and reliability at the time of abnormality such as overcharge can be ensured by providing the center pin in the hollow portion of the wound electrode assembly. However, providing the center in increases the number of parts and causes a disadvantage in terms of productivity and cost. In addition, although a further increase in battery capacity has been desired in recent years, the space for the center pin is a dead space when the center pin is provided, and it is difficult to increase the battery capacity.
Meanwhile, to prevent power loss inside a battery and reduce heat generation in a nonaqueous electrolyte secondary battery, the efficiency of collecting power from a substrate of an electrode plate is increased by increasing the width of as collector tab to increase the attachment area to the substrate of the electrode plate. However, in a cylindrical nonaqueous electrolyte secondary battery, a configuration in which the width of a collector tab is simply increased cannot be directly employed since the attachment surface of the collector tab is in a curved shape and assembly into a cylindrical battery outer would be difficult. Therefore, in an invention of a cylindrical nonaqueous electrolyte secondary battery disclosed in Patent Document 2 below, a collector tab of an electrode on the innermost circumference side is formed to have the same radius of curvature as a core bar used at the time of preparation of a wound electrode assembly.
Patent Document 3 below discloses a cylindrical nonaqueous electrolyte secondary battery in which two negative electrode collector tabs are provided on the winding start side and the winding end side of a negative electrode substrate to reduce the contact resistance between the negative electrode collector tab and a battery outer can without particularly increasing the width of the collector tab. For the cylindrical nonaqueous electrolyte secondary battery disclosed in Patent Document 3 below, an example is shown as follows: the collector tab provided on the winding end side is bent toward the winding center after preparation of a wound electrode assembly; three layers of the collector tab on the winding start side, the collector tab on the winding end side, and the battery outer can are welded and electrically connected at an inside bottom portion of the battery outer can; and a portion of a shape having elasticity is formed at a joining portion of the collector tab on the winding start side and the battery outer can in order to provide resistance against impact and vibration.